Engineers have long known that turbochargers are subjected to stresses that limit the life of the turbocharger. Specifically, the life of a turbocharger is limited by both low cycle fatigue and creep on a compressor wheel and a turbine wheel of the turbocharger. Low cycle fatigue results from acceleration of the turbocharger during load changes. When the turbocharger rotational speed increases, the centrifugal forces acting on material comprising rotors of the turbocharger compressor wheel and turbocharger turbine wheel may cause the material to expand. However, when the turbocharger rotational speed decreases, the same material may contract. The repeated expansion and contraction of the material comprising the compressor and turbine wheel rotors will cause fatigue, which may eventually lead to turbocharger failure.
Further, the life of a turbocharger is limited by creep. Creep is the slow movement of the material comprising the compressor and turbine wheels under high stress of high turbocharger rotational speeds and high inlet air or exhaust temperature. The material will deform and loose strength at an increasing rate as the stress and temperature increases. The onset temperature of creep differs among materials. Although the turbine wheel and compressor wheel may be comprised of different materials and operate at different temperatures, creep can eventually cause both the compressor and turbine wheels to fail.
Moreover, a turbocharger's resistance to creep and fatigue can be reduced by material degradation of the compressor and turbine wheels. The material properties can deteriorate due to the metallurgical changes, such as oxidation or corrosion, when subjected to high temperatures for a period of time. Although the turbine wheel and the compressor wheel may be comprised of different materials and operate at different temperatures, material degradation can contribute to failure caused by fatigue and creep in both the compressor and turbine wheels.
Because turbocharger fatigue and creep are caused by the operating conditions of the turbocharger, the life of the turbocharger is directly related to how the turbocharger is used, which is often referred to as the “duty cycle” of the turbocharger. Turbochargers are used in a variety of vehicle and stationary applications powered by internal combustion engines. Further, turbochargers that have similar applications may also be exposed to different turbocharger operating conditions depending on the duty cycle of the turbocharger. Thus, because there are many different applications and duty cycles of turbochargers, the life of a particular turbocharger can vary substantially from the average life of similar turbochargers.
Because fatigue and creep will eventually cause the turbocharger to fail, the turbocharger must be replaced or serviced prior to failure. Thus, there must be a determination of when the turbocharger will fail. Often, a representative duty cycle is used to estimate when the turbocharger should be replaced. The representative duty cycle refers to the life of a turbocharger being used in an average manner. However, because turbochargers have many different applications, using a representative duty cycle to predict the lives of various turbochargers leads to over estimation of some turbochargers' lives and under estimation of other turbochargers' lives. If the representative duty cycle over estimates the life of the turbocharger, the turbocharger will fail prior to being replaced, resulting in costly repairs and customer inconvenience. If the representative duty cycle under estimates the life of the turbocharger, the turbocharger will be unnecessarily replaced, resulting in unnecessary expense and inconvenience.
Thus, another method of determining the life of a turbocharger has been to directly monitor the rotational speed of the turbocharger and the turbine wheel inlet temperature. For instance, the turbocharger fatigue life monitor, shown in U.S. Pat. No. 6,209,390 B1, issued to LaRue et al., on Apr. 3, 2001, includes at least one sensor that measures the actual operating condition of the turbocharger, i.e., the rotational speed of the turbocharger. A central processing unit can compare the actual operating condition of the turbocharger with predetermined data to determine when service of the turbocharger is needed.
Although the method of monitoring the actual operating condition reduces the over estimation and under estimation concerns, there are additional costs involved with direct monitoring of the turbocharger operating condition. For instance, there are costs associated with the assembly and installation of turbocharger rotational speed sensors and connections between the computer processor and the sensors. Specifically, the design of the turbocharger shaft may require alteration in order to permit measurement of its rotational speed.
Moreover, the LaRue fatigue life monitor only monitors an estimated fatigue life of the turbine and compressor wheel, and does not monitor the creep life of the wheels. Depending on the duty cycle of the turbocharger, the turbocharger life may be limited by creep rather than by fatigue. Similarly, the LaRue fatigue life monitor does not consider the effect of possible material degradation when determining the fatigue life of the turbine and compressor wheels.
The present invention is directed to overcoming one or more of the problems as set forth above.